Method of preparing tin free chromium coated steel

ABSTRACT

TIN-FREE, CHROMIUM OXIDE AND CHROMIUM METAL COATED STEEL STRIP HAVING DEFECT-FREE SURFACES IS PREPARED BY SUBJECTING STEEL TO HOT-ROLLING, PICKLING, COLD-ROLLING TO FORM STRIP, CLEANING, RINSING IN AN AQUEOUS MEDIUM CONTAINING A SUBSTANCE IN MINOR CONCENTRATION THAT REMAINS ON THE STEEL STRIP AND IS CAPABLE OF REACTING WITH CARBONACEOUS MATERIAL DURING THE BATCH ANNEALING CYCLE, WINDING THE STRIP INTO A TIGHT COIL, BATCH ANNEALING, TEMPER ROLLING, CLEANING, PICKLING, AND COATING WITH A THIN FILM OF METALLIC CHROMIUM AND CHROMIUM OXIDE.

United States Patent 3,756,926 METHOD OF PREPARING TIN-FREE, CHROMIUM COATED STEEL Lewis Judson Brown, North Wales, and Joseph Edmund Lippy, Jr., Norristown, Pa., and Herbert James Payne, New Brunswick, N.J., assignors to Pennwalt Corporafion, Philadelphia, Pa. No Drawing. Filed Mar. 3, 1972, Ser. No. 231,747

Int. Cl. C23b 5/50, 9/00 US. Cl. 204-29 4 Claims ABSTRACT OF THE DISCLOSURE This invention concerns improved tin-free steel plate and more particularly relates to a method of providing electrolytically chromium-coated, and chromium oxidecoated, steel strip free of certain unacceptable surface defects and blemishes in the coating.

Steel that has been hot-rolled, pickled, cold-rolled, cleaned, batch-annealed, temper-rolled or further cold reduced, cleaned, pickled and electrolytically chromiumcoated, and chromium oxide-coated, frequently is rejected as unuseable after undergoing these expensive treatments because surface defects on the steel are visible through the chromium metal-chromium oxide coating. Moreover, interaction between the substance of the defect and the chromium coating bath will cause poor adhesion of the coating. These surface defects, which may appear to the naked eye as stains and discolorations are usually referred to in the trade as suft. The defects, which are unique to batch-annealed material, are believed to be caused by a trace amount of carbonaceous material that clings to or is associated with the surface of the strip, resists all efforts to remove it by cleaning and pickling procedures used in the mills, and is converted to graphitic material during the batch annealing process.

In our recently issued patent, US. 3,632,487, Jan. 4, 1972, we disclosed a method of preventing surface defects in electrolytically tin-coated plate, involving the use of a preanneal rinse with a particular substance in dilute solution. We have now discovered that an analogous process is operative to produce tin-free, chromium type steel (TFS CT) plate free of surface defects. Such chromium-chromium oxide coated plate has in recent years replaced much of the tinplate previously used in can production, especially for beer and carbonated beverages.

In accordance with this invention, obtaining defect free surfaces on tin-free steel strip which is ultimately coated with a finish of chromium metal and chromium oxide is ensured by treating the steel strip, prior to the batch annealing operation, with a minor amount of a substance that is capable of reacting with carbonaceous material on the strip during the batch annealing cycle to convert the carbonaceous material to volatile by-products, believed to be caron disulfide and/or carbon monoxide.

More particularly, the process of this invention embodies the steps of:

(1) hot-rolling a steel slab to a thickness of the order of about 0.08 inch;

(2) pickling the hot rolled steel in dilute sulfuric acid or hydrochloric acid and water-rinsing;

(3) cold-rolling the steel to a strip thickness of the order of about 0.009 inch;

(4) cleaning the strip in hot aqueous alkaline solution,

rinsing and scrubbing;

(5) rinsing the strip in an aqueous medium containing a substance in minor concentration which remains on the steel after withdrawal thereof from the aqueous medium and which can react with minor amounts of carbonaceous material on the steel at the temperatures encountered in batch annealing to form a volatile product (i.e., a product that volatilizes at temperatures of the annealing cycle);

(6) winding the strip into a tight coil and batch annealing said coil at about 1150 F. to about 1250 F. for from about 7 to about 10 hours in a conventional protective gas atmosphere;

(7) temper rolling or further cold reducing the strip to about 0.006 inch followed by cleaning essentially as in step No. 4, pickling, usually in dilute sulfuric acid, water-rinsing; and

(8) electro-depositing a coating of chromium metal, and forming a film of chromium oxide on the surface of the strip.

Ordinarily, the metallic chromium coating applied in step 8 will be on the order of about 5-8 mg. per sq. ft. of surface and the chromium oxide film on the order of about 14 mg./ft. The chromium and chromium oxide coatings may be applied to the steel strip by conventional techniques such as those described in the article by G. G. Kamm, A. R. Willey and N. J. Linde, Surface and Corrosion Characteristics of Tin-Free Steel-Chromium Type for Beverage Containers, J. Electrochem. Soc.: Electrochemical Technology, pp. 1299-1305, September 1969.

The active substances present in the aqueous, dilute, preanneal rinse employed in step No. 5 referred to above are generally used in concentrations of from about to about 2500 ppm. of the aqueous medium, preferably about 500 to about 1000 p.p.m. The operable substances are of two groups, both, however, within the general class of those compounds which may react with carbonaceous material on the strip steel under batch annealing conditions to form volatile products. The first group embodies sulfur-containing compounds which may react with the carbonaceous material to yield carbon disulfide. Representative of such compounds are sulfur and the alkali metal and alkaline earth metal sulfates, bisulfates, snlfites, bisulfites, thiocyanates, dithionates, thiosulfates, sulfides and polysulfides, organic sulfides, disulfides, thioethers, sulfoxides, sulfones, sulfate esters and sulfonates. Preferred materials from these classes are colloidal sulfur, ammonium sulfide, ammonium sulfate, sodium sulfate, sodium thiosulfate, ammonium xylene sulfonate, sodium sulfide, thiourea, diethylthiourea, potassium sulfate and potassium thiosulfate. The second group embodies compounds which may react with the carbonaceous material to yield carbon monoxide during the annealing. Representative of such compounds are the alkali metal and alkaline earth metal oxides and hydroxides and salts that decompose at the temperatures of the batch annealing to form said alkali metal and alkaline earth metal oxides. Preferred materials from these classes are calcium hydroxide, sodium hydroxide, barium hydroxide, potassium titanium oxalate, potassium permanganate, sodium nitrite, sodium borate and sodium bicarbonate.

We have found that treatment of the strip steel with an aqueous rinse containing a material as described above, followed by conventional batch annealing and cleaning, eliminates stain patterns which would normally show through the chromium coating and also tend to cause inadequate adhesion of the electrolytically applied coating, such defects being characteristic of the defective steel strip herein described.

The results and advantages of this invention may be demonstrated by a laboratory simulation of commercial TFS-CT plate manufacture. In such a test, six 3" x 6" panels (about 0.009 inch thick) of cold rolled steel strip known to produce typically defective TFS-CT plate as herein described are first cleaned in a conventional alkaline cleaning solution. Half of the panels (the controls) are water rinsed and dried. The other half are rinsed, and dipped in aqueous media containing one of the foregoing additives of the invention, and dried. The panels are stacked, placed in an annealing chamber, and squeezed together by placing a /2 inch steel plate on top of the stack to simulate a tightly wound coil of strip steel as formed in a commercial plant. The annealing chamber containing the panel bundle is charged to an annealing furnace and subjected to the T-l batch annealing cycle (essentially 10 hours at 1185 F. to 1230" F., requiring about three hours to reach said temperatures, and followed by about a twelve hour cooling cycle) under a protective gas atmosphere consisting of hydrogen and 95% nitrogen. After cleaning, rinsing, pickling, and rinsing, the panels are coated with a layer of chromium metal (5-8 ing/ft. equivalent to 0.30.5;/. inches) by conventional electro-coating in a typical chromium metal plating solution (M & T Unichrome chrome plating bath) and with a film of chromium oxide (1-4 mg./ft. in a representative solution of chromic acid containing caustic soda and sodium fiuosilicate additives, and finally rinsed and dried. The TFS-CT coated control panels are usually rejected because of staining, and adhesion of the coating is poor when subjected to the Scotch tape test. However, TFS-CT coated panels treated according to the invention with special pre-anneal rinse materials, as exemplified in the following table, are uniform in appearance, without flawed surfaces, and the coatings demonstrate good adhesion.

REPRESENTATIVE PREANNEAL RINSES Concentration in aqueous Preanneal rinse additive: medium in p.p.m.

(NH SO 250 '('NH SO 500 Na SO Na S 500 NH SCN 500 Colloidal sulfur 500 Na S O Diethylthiourea 1000 (NH SO and NaNO 400 and 100 Diethylthiourea and NaNO 800 and 200 Tetrahydroxy naphthalene sulfonate 1000 Sodium xylene sulfonate 1000 Thiourea 500 Thiourea 1000 KM O 500 NaOH 500 NaOH 1000 Ca(OH) 500 Ca(OH) 1000 NaNO 500 N3-2B407' Na C O Na2Ti307 Panels similarly treated prior to annealing with aqueous media containing dilute concentrations (500 to 1000 ppm.) of inoperable substances, such as sodium chloride and ammonium chloride, will normally have numerous surface defects which are visible through the chromium coatings and render the steel strip unacceptable.

We claim:

1. A process for preparing chromium-coated steel strip free of surface defects which comprises treating steel according to the following sequence of steps:

(a) hot-rolling a steel slab;

(b) pickling the hot rolled steel and water-rinsing;

(c) cold-rolling to form steel strip;

(d) cleaning the strip in aqueous alkaline solution,

rinsing and scrubbing;

(e) rinsing the steel strip in an aqueous medium containing in admixture from about to 2500 ppm. of substance that remains on the steel after withdrawal thereof from the aqueous medium and which is capable of reacting with carbonaceous material on the steel at temperatures encountered in batch annealing to form a product that volatilizes at temperatures of the annealing cycle selected from the class consisting of carbon disulfide and carbon monoxide;

(f) winding the steel strip into a tight coil, and batch annealing, whereby carbonaceous material on said steel reacts with said rinse substance and is converted to carbon disulfide or carbon monoxide which volatilizes;

(g) temper rolling or further cold reducing the steel strip, cleaning as in step (d), pickling, water-rinsing; and

(h) electro-depositing a coating of chromium metal,

and forming a film of chromium oxide on the surface of the strip.

2. The process according to claim 1 wherein the concentration of the substance in the aqueous medium of step (e) is from about 500 to about 1000 ppm.

3. A process for preparing chromium-coated steel strip free of surface defects which comprises treating steel according to the following sequence of steps:

(a) hot-rolling a steel slab;

(b) pickling the hot rolled steel and water-rinsing;

(c) cold-rolling to form a steel strip;

(d) cleaning the strip in aqueous alkaline solution,

rinsing and scrubbing;

(e) rinsing the steel strip in an aqueous medium containing in admixture from about 100 to about 2500 ppm. of material selected from the group consisting of sulfur, alkali metal and alkaline earth metal sulfates, bisulfates, sulfites, bisulfites, thiocyanates, dithionates, thiosulfates, sulfides, polysulfides, organic sulfides, disulfides, thioethers, sulfoxides, sulfones, sulfate esters, sulfonates, the alkali metal and alkaline earth metal oxides and hydroxides and salts that decompose at batch annealing temperatures to form said alkali metal and alkaline earth metal oxides;

(f) winding the steel strip into a tight coil and batch annealing;

(g) temper rolling or further cold reducing the steel strip, cleaning as in step (d), pickling, water-rinsing; and

(h) electro-depositing a coating of chromium metal,

and forming a film of chromium oxide on the surface of the strip.

4. The process according to claim 3 wherein the concentration of the material in the aqueous medium of step (e) is from about 500 to about 1000 p.p.m.

References Cited UNITED STATES PATENTS l/1972 Brown et al 20428 1/1967 Yonezaki et a1. 20441 U.S. Cl. X.R. 20434, 56 

